The present invention relates to a method of producing an automotive vehicle body part by injecting a plastics material, to a method of injecting a plastics material sequentially, to a mold for carrying out said methods, and to the plastics material parts obtained.
The fenders and external door panels of automotive vehicles constitute parts which are particularly difficult to produce from plastics material since their surface must be very good, their mechanical properties must be good and they have to resist the cataphoresis treatment which has to be carried out on the metal bodywork and structural parts of a vehicle.
Further, because they are thin, i.e., about 2 to 3 millimeters (mm) thick, while their surface area is of square meter (m2) order, the geometry of such parts does not lend itself well to injection molding, meaning that the injection conditions have to be carefully controlled unless surface defects, mainly due to irregular flow of the plastics material in the mold, can be tolerated.
The document xe2x80x9cSEQUENTIAL SOLUTIONSxe2x80x9d (H. Guyot), published in EUROPEAN PLASTICS NEWS, vol. 21, No 9, October 1994, page 29, demonstrates that it is known to improve the surface quality of such parts using sequential injection of the material to fill the cavity of the mold in successive passes.
However, that sequential injection method is suitable only for thermoplastic materials with a relatively low melting point, such as polypropylene, since the shut-off nozzles used can operate only within a limited temperature range.
In order to obtain particular mechanical properties, certain body parts, in particular fenders, are produced from an alloy of polyamide and a polyphenylene-ether which is a material which is injected at very high temperature, preferably 310xc2x0 C.
Such an injection temperature is incompatible with sequential injection carried out using the shut-off nozzles that are currently available.
There is thus a need to provide a method of injecting a plastics material sequentially at very high temperature.
The present invention aims to provide novel methods which can overcome those disadvantages.
In a first aspect, the present invention provides a method of producing an automotive vehicle body part by injecting a plastics material sequentially into a mold fed by one or more cold runners, the method consisting in filling firstly a first region at one end of the mold, then a second region adjacent to the first region, and repeating the operation consisting in filling a new region adjacent to the preceding region each time, until a last region of the mold is reached at its end opposite from the first region, the method being characterized in that each cold runner is shut off using a cold rod which is axially movable in a bore intersecting the cold runner, the longitudinal section of the rod overlaying the cross section of the runner.
Within the context of the invention, the term xe2x80x9ccold runnerxe2x80x9d and xe2x80x9ccold rodxe2x80x9d mean an unheated runner and rod, which are therefore at the temperature of the mold at the moment of injection.
The method of the invention is particularly applicable to sequential injection of an alloy of a polyamide and a polyphenylene-ether, even though this material is very sensitive to temperature variations and solidifies very rapidly just after being injected, to such an extent that if this alloy is injected into a mold via cold runners, i.e., supply runners which are not preheated at the moment of injection, a molten alloy sheath forms inside the runner, which sheath has viscosity higher than that of the remainder of the molten alloy and therefore penetrates into the mold cavity at a slower rate than the remainder of the molten alloy, producing surface defects which can result in rejection of the part if it does not satisfy minimum surface quality conditions.
The inventors of the present invention have demonstrated that contrary to expectations, the surface defects observed, in particular when injecting at a plurality of points through cold runners, disappear if the mold is filled by commencing at one end thereof and progressing towards its opposite end.
The present invention is of particular application when a plastics material is injected at high temperature, for example the alloy of polyamide and polyphenylene-ether cited above, where injection is carried out above 290xc2x0 C., preferably 310xc2x0 C.
In a preferred embodiment of the invention, the plastics material used is an alloy of the type which has to be injected at more than 290xc2x0 C., in particular an alloy of a polyamide and a polyphenylene-ether, such as the alloy sold by GENERAL ELECTRIC PLASTICS B.V. under the trade name NORYL GTX.
In order to inject the plastics material by filling contiguous regions of the mold starting from one end thereof, in one particular implementation of the invention, a mold can be used that comprises a plurality of cold runners to direct the plastics material into the mold cavity, these cold runners being shut off at the start of injection and then sequentially opened to inject plastics material into each successive region of the mold.
In this respect, the present invention has the advantage that the shut-offs used are not bulky and can be juxtaposed and be very close to one another, meaning that molds can be produced with tightly grouped sprue trees, something which was impossible with the heated shut-off nozzles used until now because of their bulk.
In a particular variation of this implementation, after one cold runner has been opened, it is individually shut off again before the next cold runner is opened, and then all the cold runners are opened to compact the plastics material and keep it under pressure as it cools.
Sequential injection is thus carried out at a plurality of points.
Given that the cavity feed runners are cold runners, causing the molten plastics material to stand still would be expected to make it tend to solidify in the runners, but tests carried out by the inventors have shown that it is possible for each runner to be opened, shut off and opened again without producing a defect in the part obtained.
In a preferred variation, the rod used as a shut-off also acts as an ejector to unmold the plastics material sprue tree originating from the cold runners during molding of the plastics material part.
The present invention also provides a method of injecting plastics material sequentially into a mold fed by cold runners, to produce an automotive vehicle body part, the method being characterized in that it consists in providing the mold with bores intersecting the cold runners and with axially movable cold rods in said bores, the longitudinal section of each rod overlaying the cross section of the corresponding runner, in shutting off the cold runners using the rods at the start of injection, and in opening them individually by displacing the rods in the bores.
This method has the advantage of being capable of being carried out in simple manner, avoiding the need for shut-off nozzles.
Further, the method of the invention enables sequential injection to be carried out in molds having cavities that do not permit shut-off nozzles to be housed.
An example of an application of sequential injection which can be mentioned is the above-described method consisting in filling the mold, starting from one of its ends and going towards its opposite end, but clearly the sequential injection method of the invention is not limited to this application.
The invention also provides an injection mold for carrying out the above-described methods, the mold being characterized in that it comprises, as a shut-off for a cold runner, a rod with a longitudinal section which overlays the cross section of the cold runner, the rod being axially movable in a bore intersecting the cold runner between a shut off position in which it traverses the cold runner and an open position in which it is retracted from the cold runner and leaves the cross section of the cold runner open.
Such a mold has the advantage of being particularly economical to manufacture and maintain.
From the manufacturing viewpoint, the use of simple cold rods to shut off the cold runners reduces the cost of the mold by a factor of five or seven. Regarding maintenance, the absence of a heating block in the mold eliminates cooling time and heating time which generally prolong downtime.
In a preferred variation, the rod also acts as an ejector for unmolding the plastics material sprue tree originating from the cold runners on unmolding the plastics material part.
This variation provides a mold which is very adaptable in use since this mold can also be used to carry out the invention, i.e., shutting off the cold runners by the ejectors then ejecting the part obtained, and also to carry out molding without shutting off the cold runners, the ejectors then acting solely to eject the part obtained.
In a particular embodiment, the rod is mounted on an independent actuator which allows it to be displaced in the bore and to shut off the cold runner while the mold is closed.
In other words, this actuator enables to rod to carry out both of the shut-off and ejection functions, the shut-off function being accomplished when the mold is closed and the independent actuator urges the rod into the bore to intersect the cold runner, while the ejection function is accomplished following injection, after the rod has been returned to the normal position by the independent actuator, when the rod acts while unmolding the complete part to eject the sprue tree that results from the cold runners being filled.
The present invention also provides plastics material parts obtained by carrying out the methods described above.